This invention relates to a method of producing a metal sheet product. It relates particularly to a method for producing a steel sheet that is hot-dip coated with zinc, tin, terne, aluminum, aluminum-zinc alloys and lead.
A hot-dip coating of zinc or other protective metals on steel sheet is produced by passing a suitably prepared cold rolled steel sheet through a bath containing the molten coating metal, such as a molten zinc. The coating of steel sheet is usually done on a continuous basis using one of several well developed coating processes.
One of the most common of the hot-dip coating processes in use today for coating steel sheet with zinc or zinc-aluminum alloy is the "Non-Oxidizing Process" as described in U.S. Pat. No. 3,320,085 assigned to Selas Corporation.
The "Non-Oxidizing Process" for galvanizing sheet steel includes passing a steel strip through a washing or pickling process to remove oil and dirt, followed by a water rinse which leaves a thin film of oxide coating on the steel strip. The strip is then passed through a gas fired furnace containing a reducing atmosphere which causes a reduction of the oxide coating on the surface of the steel strip and the formation of a tightly adherent impurity-free iron layer on the steel strip. The strip remains contained within the reducing atmosphere in the furnace and furnace enclosed snout until it is immersed in a molten zinc bath maintained at a temperature of about 850.degree. F. (456.degree. C.). The strip is then air cooled, resulting in a bright spangled surface characterized by several thin iron-zinc intermetallic layers between the steel base and a relatively thick overlay of free zinc.
One area of interest for hot-dip coating research has been the formability of the hot-dip coated sheet. Many applications for hot-dip coated sheet require a fairly ductile product and practices described in U.S. Pat. No. 4,287,008 to Torok, et al.; U.S. Pat. No. 3,297,499 to Mayhew; U.S. Pat. No. 3,111,435 to Graff, et al.; U.S. Pat. No. 3,028,269 to Beattie, et al. and U.S. Pat. No. 2,965,963 to Batz, et al. are all directed to improving the ductility of hot-dip coated steel sheet after it has been coated. These prior patents are directed to the practice of post annealing the entire coated sheet to effect changes in the base steel without changing the corrosion protection characteristics of the coating itself.
One large market for hot-dip coated steel sheet is the metal building industry where the coated steel sheet is used for roofing and siding panels on commercial and residential buildings. The hot-dip coated steel sheets used for roofing are usually corrugated or roll formed for strength and stiffness, and joined together at their edges by a continuous weatherproof lockseam. The sealing lockseam requires sufficient ductility in the steel sheet to allow for rather severe bends as the lockseam is formed. As a result, the metal building industry would like a highly corrosion resistant hot-dip coated steel sheet product exhibiting both good strength and stiffness characteristics, as well as suitable side edge ductility to facilitate edge forming at either the shop or in the field. Other product applications of such hot-dip coated steel sheets would be large containers or doors. These large products also require the hot-dip steel sheet to have good strength and stiffness, resistance to corrosion but also good formability at the edges of the sheet for making seams and connections.